The present invention relates generally to asynchronous transfer mode (ATM) networks and other types of packet-based communication systems, and more particularly to techniques for transmitting voice and telephony service information over such systems.
An ATM communication system provides high-speed, low-delay switching of voice, data, video and other types of user information traffic. In an ATM system, the user information traffic is separated into fixed-length 53-byte cells. Each ATM cell typically includes a 5-byte header and a 48-byte payload. The header incorporates a virtual channel identifier (VCI) and a virtual path identifier (VPI) associated with the corresponding cell. The VCI and VPI together specify a virtual connection (VC) which is established when a user requests a network connection in the ATM system. Additional details regarding these and other aspects of ATM systems can be found, for example, in the ATM Forum, xe2x80x9cATM User-Network Interface Specification,xe2x80x9d Version 3.1, September, 1994, and in Martin de Prycker, xe2x80x9cAsynchronous Transfer Mode: Solution for Broadband ISDN,xe2x80x9d Ellis Horwood, New York, 1993, both of which are incorporated by reference herein.
The ATM standard includes a number of ATM Adaptation Layers (AALs), each specifying different types of connections in an ATM system. One such AAL is known as AAL Type 2, or simply AAL 2. AAL 2 includes a Common Part Sublayer (CPS) which communicates with the ATM layer, and a Service Specific Convergence Sublayer (SSCS) that operates between the CPS and a communication service layer. The purpose of the SSCS is generally to convey narrowband voice and telephony service information, such as voice, voiceband data, or circuit mode data, associated with various voice and telephony communication services. The SSCS specifies packet formats and procedures to encode different call-related data streams for bandwidth-efficient transport by AAL 2. The SSCS accommodates known techniques of low rate audio encoding, silence compression, and facsimile modulation/demodulation. The CPS provides multiplexing functions which allow many calls to be sent over a single ATM connection. The single ATM connection therefore acts as a trunk group for transmission between two points of access. Additional details regarding AAL 2 and the SSCS can be found in, for example, xe2x80x9cAAL Type 2 Service Specific Convergence Sublayer for Trunking,xe2x80x9d ITUxe2x80x94Telecommunication Standardization Sector, Draft Recommendation 1.366.2, Temporary Document 24-E (PLEN), June 1998, which is incorporated by reference herein.
A significant problem with the existing SSCS in AAL 2, and with similar mechanisms in other packet-based communication systems, is the failure to provide efficient, low-overhead support for a variety of voice and telephony functions, including silence elimination, comfort noise generation and rate control.
The invention in an illustrative embodiment provides a low-overhead ATM Adaptation Layer 2 (AAL 2) Service Specific Convergence Sublayer (SSCS) for providing Voice and Telephony over ATM (VTOA). An SSCS in accordance with the invention provides the ability to support the delineation of silence intervals, e.g., for silence elimination, as well as other voice and telephony functionality, using a minimal number of bits.
In the illustrative embodiment, an ATM AAL 2 SSCS transmitter associates a holdover indicator with each of a number of packets generated in a corresponding voice and telephony service layer. The holdover indicator may be a holdover bit in a User-to-User-Indication (UUI) field of a Common Part Sublayer (CPS) header of the corresponding AAL 2 connection. The holdover indicator may specify, for example, whether a speech detector used for providing silence elimination in the transmitted information is in a holdover mode. The indicator can thus be used by an SSCS receiver to determine if a period of silence has begun in the transmitted information, so as to facilitate the generation of comfort noise by the corresponding voice and telephony service layer. The holdover indicators can also be used to provide rate control in an AAL 2 CPS transmitter. For example, the CPS transmitter could drop packets having a holdover indicator of a specified value if a maximum bandwidth of a given transmission connection is exceeded. In other words, the CPS transmitter could use the holdover indicator to provide transmission priority to packets containing speech or other specified types of audio information.
Although the illustrative embodiment is directed to the transmission of voice and telephony information over an ATM connection using AAL 2, the invention can also be implemented in other types of packet-based communication systems including, for example, Frame Relay systems and Voice-over-IP systems. In addition, the invention can be applied to other types of audio information. These and other features and advantages of the present invention will become more apparent from the accompanying drawings and the following detailed description.